Thin-film transistor (TFT) devices, widely used in switching or driver circuitry for electro-optical arrays and display panels, are conventionally fabricated on rigid substrates. Typically, these substrates are of glass or silicon. The TFT devices are formed onto the substrate using a well-known sequence of deposition, patterning and etching steps. For example, amorphous silicon TFT devices are formed in a process that uses deposition, patterning, and etching of metals, such as aluminum, chromium or molybdenum, of amorphous silicon semiconductors, and of insulators, such as SiO2 or Si3N4, onto a substrate. The semiconductor thin film is formed in a series of patterned layers having typical thicknesses ranging from several nm to several hundred nm, with intermediary layers having thicknesses on the order of a few microns.
The requirement for a rigid substrate has been based largely on the demands of the fabrication process itself. Thermal characteristics are of particular importance, since TFT devices are fabricated at relatively high temperatures. Thus, the range of substrate materials that have been used successfully is somewhat limited, generally to glass, quartz, or other rigid, silicon-based materials.
There is considerable interest in forming TFT devices on flexible substrates, such as metal foil and plastic substrates. However, there are a number of practical problems that must be addressed for handling and processing flexible substrates. Fabrication equipment for thin-film components, designed for use with rigid substrate materials, requires that the receiving surface of the substrate be very flat and well-supported. To achieve the needed flatness and support, a flexible substrate must be mounted on a carrier of some type during fabrication. Rigid glass carriers have been used, for example. Following fabrication processing, the substrate can be removed from its carrier. Conventional solutions that have been used for mounting the carrier include the use of adhesives, adhesive tape, and various clamping schemes. U.S. Patent Application Publication 2007/0091062 discloses various electronic devices having plastic substrates coated onto the whole surface of rigid carriers during device fabrication, without patterning the surface into release and non-release areas. Although, in prototype modeling, these conventional solutions have shown the feasibility of forming thin-film circuitry on a flexible substrate, the present applicants have found that they fall short of what is needed for volume manufacture, require considerable handling of the flexible substrate, and impose some constraints on minimum substrate thickness.
Various other techniques for supporting a flexible substrate on a carrier have been developed. For example, reference is made to U.S. Patent Application 2008/0026581 (Ser. No. 11/461,080 ) filed Jul. 31, 2006 by Tredwell et al. entitled FLEXIBLE SUBSTRATE WITH ELECTRONIC DEVICES FORMED THEREON; and U.S. Patent Application 2008/0090338(Ser. No. 11/538,173 ) filed Oct. 3, 2006 by Tredwell et al. entitled FLEXIBLE SUBSTRATE WITH ELECTRONIC DEVICES AND TRACES, incorporated herein by reference.
The fabrication process for the TFT may require temperatures in the range of 125-300 degrees C. or higher. One particular problem with metal substrates relates to expansion and contraction of materials under temperature extremes, normally expressed in terms of Coefficient of Thermal Expansion (CTE). Metallic materials differ significantly in CTE from glass, for example. The significant difference in CTE between metals and glass results in excessive stress that can shatter a glass carrier or can cause a metal substrate to release from a glass carrier prematurely, losing its dimensional stability.
Although there has been great interest in developing flexible metallic substrates, the incompatibility of metal foils with a conventional glass carrier over a broad temperature range imposes some constraints on substrate material type. There is a recognized need for methods that allow a non-metallic carrier to support a flexible metal substrate during electronic component fabrication.